1. Field of the Invention
This invention relates to a surface-coated sintered hard body and more particularly, it is concerned with surface-coated cemented carbide or carbonitride alloys having at least one boron-containing coating layer, which exhibit good performances.
2. Description of the Prior Art
The so-called surface-coated cemented carbides or carbonitrides articles having a thin coating layer of TiC or TiN possessing excellent wear resistance have widely been put to practical use as a cutting tool having more excellent toughness as well as more excellent wear resistance than the prior art cemented carbides or carbonitrides articles. These cemented carbides or carbonitrides articles consist of at least one of carbides and carbonitrides of at least one element selected from titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten, cemented by at least one metal from the iron group. The TiC-coated cemented carbides or carbonitrides have more excellent cutting properties than the prior art cemented carbides or carbonitrides, but the cutting properties have hitherto been improved only to some extent because titanium carbide meets with a marked lowering of hardness at a high temperature and lacks in oxidation resistance. In order to overcome these disadvantages, cemented carbide alloys coated with ceramic materials such as Al.sub.2 O.sub.3 and ZrO.sub.2 have been proposed. However, the adhesive strength of such a ceramic layer to a cemented carbide alloy is not sufficient for practical use and, therefore, it has been proposed to use at least one of carbides and nitrides of metals such as titanium, zirconium, etc. between the ceramic layer and cemented carbide alloy. In fact, this laminated article has really been used. When, for example, a titanium carbide and/or nitride is used as a layer under an Al.sub.2 O.sub.3 layer as described above, however, the low oxidation resistance at a high temperature, which is one of the above described two disadvantages of the TiC-coated cemented carbides, is somewhat improved, but the lowering of the hardness at a high temperature as a whole is only improved partly. That is to say, even if the surface is coated with a ceramic material whose hardness is not so lowered even at a high temperature, such lowering of hardness of the coated layers as a whole is unavoidable as far as TiC is used under the ceramic layer.